Spinal adrenergic mechanisms regulating sympathetic outflow to blood vessels

The role of descending spinal catecholamine (CA)-containing fibers in cardiovascular regulation was examined. Monoamine-containing fibers were visualized by fluorescence histochemical methods at the cervical, thoracic, and lumbar spinal segments. Two major groups of descending CA-containing axons were located in the lateral funiculus at the midcervical spinal level. At the thoracolumbar spinal segments CA-containing terminals were concentrated mostly around cells of the intermediolateral nucleus. In addition a distinct CA-containing fasciculus was observed at the level of the sympathetic nucleus which appeared to course toward the opposite site of the cord. To evaluate the role of the efferent CA-containing fibers, changes in heart rate, arterial pressure, femoral blood flow, and calculated peak vascular resistance were elicited by electrical stimulation of selected sites in the midcervical spinal cord. Although changes in femoral flow and arterial pressure were evoked from many midcervical sites, there was a distinct correlation between the magnitude of peak femoral resistance elevation and density of CA-containing axons activated. Furthermore, efferent spinal vasoconstrictor outflow to the hindlimbs crosses below the cervical level. This observation is in good agreement with fluorescence microscopy studies revealing CA-containing fibers which appear to decussate. Pharmacological studies also were undertaken to evaluate transmission in spinal vasopressor pathways. In these studies alpha-adrenergic receptor antagonists and agonists were perfused into the spinal subarachnoid space. Heart rate, arterial pressure, femoral flow, and femoral resistance responses elicited from efferent spinal pathways were significantly attenuated following superfusion of the spinal cord with the alpha-antagonists BE-2254 (HEAT) and phentolamine. It was shown that inhibition of the centrally evoked responses was due to actions of the alpha-antagonists at a spinal locus and not due to peripheral vascular alpha-receptor blockade. Spinal perfusion with the alpha-receptor agonist norepinephrine potentiated cardiovascular responses. Likewise, loading with the norepinephrine precursor 3,4-dihydroxy-L-phenylalanine (L-dopa) enhanced vasoconstrictor responses evoked in the cross-perfused hindlimb of p-chlorophenylalanine-pretreated cats. These observations support the idea that bulbospinal CA-containing fibers relay excitatory impulses to preganglionic vasoconstrictor neurons and that spinal alpha-adrenergic receptor activation is necessary for transmission of this information.